@Tomos: Sorry, but I thought Google's blocking this [API] was already quite clear from the Picasa blog:
-IainB
clear from their blog as of March 26th 2018 (it may have been clear earlier, but I dont think so) -- either way, I, and lots of other people using Picasa this way didnt know about it.
There's nothing to stop you from using Picasa as an image management tool/database, and shipping up the photos to Google Photos, as required. (I had thought that was what you were intending doing.)
-IainB
the nice thing about Picasa is you can create as many albums as you like:
Family photos are a good example -- using Picasa's face-recognition 'face-tags' I created albums for each family within the extended family; an album for my mother's family; an album for each brother and sister; for each aunt/uncle; etc. They're all subsets of the full collection of photos. AFAICS these albums are now useless (because I hadnt synced them yet).
I am unable to reproduce them in google photo without starting from scratch (it doesnt recognise Picasa's face-tags). I guess I'll just copy them from the Picasa album to a new local folder and offer them as zipped downloads -- not so accessible, especially to non-computer-literate people.
Although...I wonder if I upload an image twice to google photo -- to two different albums, will it recognise that it has the images already, and just 'tag' the copy it has already with the new album.